Network Working Group C.M. Lonvick
Internet-Draft May 11, 2011
Intended status: Informational
Expires: November 12, 2011

A Taxonomy on Private Use Fields in Protocols


The fields in protocols that are reserved for private use have been purposefully unregulated. This document attempts to provide some classifications for the way that private use fields have been used in protocols developed in the IETF.

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Table of Contents

1. Introduction

Simply put, communications protocols are standardized ways for computing entities to convey information. Within each communications protocol, there must be standardized pieces of information that will be communicated, and there may be non-standardized information that can be communicated.

Time Protocol [RFC0868] is an example of a protocol that only conveys standardized information. There is no way to add anything other than what is specified in the document. On the other hand, DoD standard Transmission Control Protocol [RFC0761] does have "options" but they must be registered through the IANA [IANAtcp], which does not leave any room for optional information supplied by equipment vendors, network operators, or experimenters. Finally, Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4) [RFC3925] does allow for vendor specific options.

If a network operator wanted to add specific information to the Time Protocol [RFC0868], they could modify the code of the senders and receivers and run this within their own domain without any problems. However, if an equipment vendor wanted to include information specific to their equipment, they would have to ensure that all senders and receivers within all network domains would either accept the change in the protocol, or would not have problems with it. As a final case, if several equipment vendors desired to add equipment-specific information to this protocol, they would have to take great care that only their own receivers would accept information from their own transmitters. An extension to that would be that if one equipment vendor would like to transmit or receive the same information that another vendor is using.

For the case of TCP [RFC0761], standard options are expected; senders may use them and receivers may be configured to act upon that information, or to ignore it. If an experimenter wants to add an option, they will have to create a new IETF RFC with the details, or obtain approval from the IESG to have the IANA add to the registry [IANAtcp]. Similarly, if equipment vendors Foo and Bar were to have a need for a similar option within TCP, they would each have to go through the process to add to the registry. On the other hand, if a properly crafted multipurpose, private use option were to be registered, such as in the case of multiple vendor instances within Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4) [RFC3925] then vendors and experimenters would each be able to use it for their own purposes as long as all network participants can easily differentiate between the entities using the option.

This document explores the various ways that protocols may allow optional information using fields designated as "private use" to be included in protocols, without disrupting the desired harmony of the network. [RFC4735]

2. Origins of the Private Use Name Space

Guidelines for Writing an IANA Considerations Section in RFCs [RFC2432] describes that values of specific name spaces may either be registered with the IANA, or not. In most cases, there are well defined values for name spaces. However, as the document explains, not all name spaces require centralized administration.

In that document, it seems to be assumed that private use name spaces will be domain specific and it will be up to the administrators of any domain to avoid conflicts. The first example given about private use name spaces refers to Dynamic Host Configuration Protocol [RFC2131] and presumably DHCP Options and BOOTP Vendor Extensions [RFC2132]. In this the example states that "site-specific options in DHCP have significance only within a single site". As noted below this became a problem that was rectified in a later revision of DHCP.

Later works identified a need to place a scope on private use name spaces. The second example of private use name spaces in [RFC2432] is from STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES [RFC0822] which describes X- headers. Again, there is no effort made to control the name space. It appears however that the users of X- headers have self-organized; most consistently use features that are universally useful and many have incorporated identifiers for useful features that may overlap.

3. Nomenclature

In this document, the following words are defined to prevent ambiguity. Some of these words have not been used in the referenced works but their meanings can be ascertained.

     [timeQuality tzKnown="0" isSynced="0"]
      <165>1 2003-10-11T22:14:15.003Z
      evntslog - ID47 [exampleSDID@32473 iut="3" eventSource=
      "Application" eventID="1011"] BOMAn application
      event log entry...


4. Examples of Successful Private Use Options

This section contains a review of RFCs that allow the use of private use options. There seem to be three ways to address the name space: via a global origin, via a truncated numerical origin, and via a namespace based upon the domain name.

4.1. SNMP

Likely, the first private use option was defined in the Structure and Identification of Management Information for TCP/IP-based Internets [RFC1155] which was first used in A Simple Network Management Protocol [RFC1067] (SNMP). The structure of management information (SMI) has been updated and is currently defined as the Structure of Management Information Version 2 (SMIv2) [RFC2578].

SMI is a well described tree of OBJECT IDENTIFIERs. It has an origin and a path for defined object identifiers which this document describes as standard options. It also allows for experimental and vendor specific object identifiers, which are described as private use options in this document. The IANA maintains a registry of these Network Management Parameters [IANAsmi].

The Internet subtree of experimental OBJECT IDENTIFIERs starts with the prefix:, and the Internet subtree of private enterprise OBJECT IDENTIFIERs starts with the prefix: This is followed by a Private Enterprise Number [IANApen] (PEN) and then the OBJECT IDENTIFIERs defined by that enterprise.

While this is very practical and practicable for SNMP, fully qualified SMIs do not lend themselves well for other uses as a generic private use option.

4.2. Private Enterprise Number

Rather than using the entire SMI, protocol engineers started using just the Private Enterprise Number [IANApen]. This reduces the length of the identifier but continues to provide an identifier through a globally unique name space. This subsection provides examples of how the PEN has been used to provide private use options.

4.2.1. RADIUS

The Remote Authentication Dial In User Service (RADIUS) [RFC2058] specification documented how to use just the PEN (without the rest of the SMI path to the root) to allow "vendors" to articulate their own options. In this document, these are called Vendor-Specific Attributes (VSA).

The updated RADIUS document, [RFC2865], gives guidance for using the VSA.

There are many attributes defined in RADIUS [RFC2058] which may be considered to be standard variables. Each of these attributes are specified within a "type length value" (tlv) container. For this protocol, the VSA "type" is a specific numerical value which separates it from other attributes. Type 26 (decimal) denotes a VSA, and the PEN starts the "value" which should then include a subsequent nested tlv so the vendor may enumerate their own options within the field.

4.2.2. Mobile IP

Mobile IP Vendor Specific Extensions [RFC3115] defines two extensions that can be used for making organization specific extensions by vendors/organizations for their own specific purposes for Mobile IP [RFC2002]. Mobile IP has been revised several times and is currently specified in IP Mobility Support for IPv4, Revised [RFC5944].

In this specification, two tlv's have been defined to contain private use options. These are called Vendor/Organization Specific Extensions (VSE). [RFC2002] which states:

Having two VSEs of this nature for private use options is consistent with the original Mobile IP specification

4.2.3. DHCP

The introduction to Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4) [RFC3925] states:

This meant that Dynamic Host Configuration Protocol [RFC2131] specified that there was one instance of the vendor type, and the receiver used that name space to set the scope for the fields in the vendor-specific information option.

This situation was resolved with the publication of Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4) [RFC3925] which states:

That specification ([RFC3925]) then used the PEN [IANApen] to defined a unique name space for private use options in this protocol. Similar to other protocols of this era, tlv containers were used.

4.2.4. IPFIX

Recording IP flow information is the subject of the Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information [RFC5101]. Since flows may be created and concluded very quickly, processing speed is essential to this protocol. As such, it is a protocol based upon binary fields that can be interpreted quickly.

Even though the protocol is binary oriented rather than character oriented, it still uses the PEN to identify the origin of the private use option. All options are identified as Information Elements in this specification. The first field of each Information Element is a single bit identified as the Enterprise bit, which is defined as follows:

The specification goes on to say:

4.2.5. Syslog

The Syslog Protocol [RFC5424] also uses the PEN to uniquely qualify the name space for a private use option. Standard options do not contain the "@" character. Private use options must have the PEN following the "@" character. This allows a vendor or experimenter to have overlapping name spaces which the PEN will then uniquely identify. For example a standard option is tzKnown which may only have associated values of "0" and "1". However tzKnown@32473 may have any value assigned to it by the owner of enterprise number 32473.

Syslog transport receivers are supposed to accept all correctly formatted Syslog messages. Unlike RADIUS, the receiving Syslog application does not have to have immediate knowledge of all variable options to continue operations. If a private use option is not immediately known to the receiving application, it may still store the message and an Operator or Administrator may look it up at a later time if they are really interested.

4.3. Character strings

The Secure Shell (SSH) Protocol Architecture [RFC4251] uses character strings rather than PENs. Similar to Syslog, but actually predating it, standard options must not have the "@" character in them. Private use options will have an origin identifier preceding an "@" character followed by a name space field. For example, in The Secure Shell (SSH) Connection Protocol [RFC4254] SSH channels may be opened by specifying a channel type when sending the SSH_MSG_CHANNEL_OPEN message. Standard options for the channel type include "session" and "x11". A private use option for a channel type could be "".

Obviously, these character strings are domain names [RFC1034] [RFC1035]. This is specified in The Secure Shell (SSH) Protocol Architecture [RFC4251]. Generally, the guidance given is that if a private use option of this nature is not understood it is to convey to its peer an error code.

5. Characteristics of Useful Private Use Options

This section summarizes the observed characteristics of private use options that are successful and deployed.

There seem to be three characteristics of successful private use options.

5.1. Source of Authority

A private use option requires a path to an origin that has the authority to create and maintain the option. As shown above, this referent should be unique, and not dependent upon local interpretation.

In the case of SNMP [smi], the globally unique origin is the International Standards Organization [ISO] which is accredited by the United Nations to maintain this structure. However, the namespace resolves to the PEN [pen] which does not seem to be globally unique.

The PEN [pen] is sourced by the Internet Assigned Numbers Authority (IANA). While there seems to be no reports of problems using PENs, PENs do not seem to be unique. It appears that some private enterprises have registered multiple PENs, and that there are some private enterprises whose PENs are no longer serviceable, mostly because they have been acquired by other companies, or they have gone out of business.

Domain names have similar problems as they can be more ephemeral than eternal. The top level domains are maintained by the Internet Corporation for Assigned Names and Numbers [ICANN] however the specific names are assigned much more locally. Unlike PENs that become unserviceable when their owning organization goes out of business, domain names that fall into disuse may be acquired and used by entirely different organizations. Again however, like the use of PENs there have not been any problems reported from this.

5.2. Focus of the Name Space

Once the source of authority is established, an actual option, or options, must be specified.

In the case of SNMP [smi], after the vendor identifier (the PEN) in the management information base (MIB) a vendor can create many different trees to identify objects. This may result in a very large number of object identifiers which are called private use options in this document.

The PEN [pen] can used in myriad ways.

Finally, in the SSH protocol [ssh], the focus of the option is dependent upon context. For example, can only be used when negotiating ciphers, while can only be used when negotiating channel types per the examples in [RFC4250].

5.3. Value of the Option

The value of each private use option must be extensible but bounded. (I'd write more but see Section 7.)

6. Issues to Consider

Private use options are useful to the community and are gaining popularity. This is a way to allow vendors, network operators, and experimenters to convey dynamic information without going through a rigorous process to register each variable.

There is no "one size fits all" mechanism. The use of a very specific and fixed format works very well for IPFIX which requires speed in processing. On the other hand, the open nature of the private use options in Syslog is appropriate for that protocol.

The structure of the SMI is uniform and globally unique, but the pen is not up to date. It appears that the people listed as the contacts for some of the organizations have moved on, and that some of the organizations have either been acquired by other organizations or have gone out of business. It also appears that some organizations have registered multiple PENs. The author is really not sure why they would need to do that so any clue here would be appreciated.

Using the PEN with an extension or even with an entire SMI may be constrained to fit nicely into a binary oriented protocol that has strict field lengths. Using domain names, as SSH does, may also be fit within binary protocols but since domain name lengths vary, tlv's or some other mechanism may need to be used to establish the boundaries of fields.

RADIUS dictates what to do if a private use option is not understood by a receiver. Mobile IP Vendor Specific Extensions [RFC3115] builds on that by letting a receiver know if it must process the entire packet containing a private use option, or if it may ignore a private use option that it doesn't understand but yet continue processing the rest of the packet. On the other hand, Syslog does not require a receiver to understand anything about any private use options. It is expected that receivers that do understand the private use options will be able to take actions more appropriately based on the information they receiver.

From that, it is important to define in protocol specifications what actions to take when a receiver receives a frame with a private use options.

7. Authors Notes

This section will be removed prior to publication.

Now that I've written it, I'm thinking that I should pivot the "characteristics" section. I think it would be easier to explain source/focus/value on a per protocol basis rather than each with examples within each protocol. That will be done in -01. :-)

I need to revise the Issues section. I'd like to write a lot more description there but I want this version out the door first.

8. Security Considerations

This document reviews ways that options are being used in various protocols. As such, there are no security considerations inherent in this document.

Readers and implementers should be aware of the context of implementing options in their own protocols.

9. IANA Considerations

This document does not propose a standard and does not require the IANA to do anything.

10. Acknowledgments

The idea for documenting this came from questions asked in the SIP-CLF Working Group and the author is grateful for the discussion around this topic.

The following people have contributed to this document. Listing their names here does not mean that they endorse the document, but that they have contributed to its substance.

David Harrington, Dan Romascanu and Bert Wijnen.

11. References

[IANAtcp] Internet Assigned Numbers Authority, "IANA Transmission Control Protocol (TCP) Parameters, TCP Option Kind Numbers", 2011.
[IANAftp] Internet Assigned Numbers Authority, "IANA FTP Commands and Extensions", 2010.
[IANAslg] Internet Assigned Numbers Authority, "IANA syslog Parameter", 2010.
[IANAsmi] Internet Assigned Numbers Authority, "Network Management Parameters", 2011.
[IANApen] Internet Assigned Numbers Authority, "IANA PRIVATE ENTERPRISE NUMBERS", 2011.
[wpProt] Wikipedia - the Free Dictionary, "Wikipedia entry for communication protocol", 2011.
[ISO] International Standards Organization, "International Standards Organization", 2011.
[ICANN] Internet Corporation for Assigned Names and Numbers, "Internet Corporation for Assigned Names and Numbers", 2011.
[RFC0761] Postel, J., "DoD standard Transmission Control Protocol", RFC 761, January 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[RFC0822] Crocker, D.H., "Standard for the format of ARPA Internet text messages", STD 11, RFC 822, August 1982.
[RFC0868] Postel, J. and K. Harrenstien, "Time Protocol", STD 26, RFC 868, May 1983.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.
[RFC1067] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple Network Management Protocol", RFC 1067, August 1988.
[RFC1155] Rose, M. and K. McCloghrie, "Structure and identification of management information for TCP/IP-based internets", STD 16, RFC 1155, May 1990.
[RFC2002] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.
[RFC2058] Rigney, C., Rubens, A.C., Simpson, W.A. and S. Willens, "Remote Authentication Dial In User Service (RADIUS)", RFC 2058, January 1997.
[RFC2432] Dubray, K., "Terminology for IP Multicast Benchmarking", RFC 2432, October 1998.
[RFC2578] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
[RFC3115] Dommety, G. and K. Leung, "Mobile IP Vendor/Organization-Specific Extensions", RFC 3115, April 2001.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)", RFC 3925, October 2004.
[RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection Protocol", RFC 4254, January 2006.
[RFC4735] Taylor, T., "Example Media Types for Use in Documentation", RFC 4735, October 2006.
[RFC5101] Claise, B., "Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information", RFC 5101, January 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5612] Eronen, P. and D. Harrington, "Enterprise Number for Documentation Use", RFC 5612, August 2009.
[RFC5944] Perkins, C., "IP Mobility Support for IPv4, Revised", RFC 5944, November 2010.

Author's Address

Chris Lonvick 213 El Rancho Grande Kerrville, Texas 78028 US EMail: